This research is directed at a study of the development of five response domains during the second and third year of life. Response domains are: memory, symbolic play, language, conceptualization, and social interaction. Two cohorts of children are followed monthly in a longitudinal design. One group is followed from 13 to 22 months, the second from 20 to 29 months of age. Preliminary results indicate (1) little covariation among the five domains with respect to level of functioning or rate of growth, suggesting no general competence factor during this period, (2) children under two are capable of rational thought and language learning seems to be due, in part, to inferences the child makes about the meaning of words.